Much research and development in thermoplastic resin films has been made by many researchers, businesses, and the like. Thermoplastic resin films have comparatively inexpensive raw materials, and also have excellent mechanical properties, chemical resistance, transparency, moisture permeability, and other such properties. These films therefore have a wide range of application, including packaging, miscellaneous goods, agriculture, industry, food products, medicine, and other fields.
There have recently been many examples in which thermoplastic resin films are used in the field of optics. Examples of thermoplastic resins include polycarbonate, cyclic polyolefins, polyethylene, polypropylene, and the like, but polycarbonate and cyclic polyolefins in particular have comparatively satisfactory transparency, and unstretched products are suitably used in light-guiding plates, anti-glare plates, starting materials for retardation films that have not yet been stretched, and the like. These films can be endowed with optical anisotropy (orientation) when subjected to stretching (uniaxial stretching or biaxial stretching). A film created from a thermoplastic resin endowed with this manner of orientation can be suitably used as a retardation film, which is used in a liquid-crystal display (LCD) or the like.
Various methods have been heretofore known and implemented for producing such thermoplastic resin films. Examples of methods that have been industrially used to produce thermoplastic resin films generally include solution casting, wherein a resin solution obtained by dissolving a resin in a solvent is cast and formed into a glass plate or the like (for example, see Patent Document 1); T-die extrusion, wherein a melted resin is extruded by an extruder and then cooled by a chill roll to form a film (for example, see Patent Document 2); tube extrusion, wherein a melted resin is extruded by an extruder into the shape of a tube (for example, see Patent Document 3); inflation extrusion, wherein a resin extruded into a tube shape is molded while air pressure is applied to the insides; and other such methods.
Particularly, to ensure a high rate of stretching, adjust the rate of stretching, and improve productivity, the following methods have been proposed. One is a method for opening up numerous split mandrels and stretching pipes or tubes (see Patent Document 6). Another method is proposed as a method for maintaining a stable operation. In this method, when an inflation film is molded, the constricted parts of the bubble formed by the film extruded from the lip of the inflation die is brought into contact either with a compressible cylindrical stabilizing member that is attached to the top center of a die and whose outside diameter is adjusted by air pressure, or with a coil spring wound in multiple stages around the outer periphery of this compressible cylindrical stable member, and the bubble is then expanded and pulled out (for example, see Patent Document 7). Another method proposed for producing high-quality films and improving efficiency is a method for producing a tubular plastic film wherein the film is extruded into a tube, and the diameter of the tube that has passed over a conical body is increased after the tube exits the extruder and before the tube reaches the pinching rollers (see Patent Document 8).
Recently, it has been proposed that stabilization means can be used to create a thin resin film product having a uniform and smooth surface, wherein nonuniformities in the thickness of the tubular film can be greatly reduced. It has also been proposed that it is possible to create an oriented, high-quality resin film product that has minimal thickness nonuniformities and stretching nonuniformities by passing the film through a stretching part and holding part formed with a porous mandrel or the like, and stretching the film there (for example, see Patent Documents 9 and 10).
[Patent Document 1] Japanese Laid-open Patent Application No. 5-239229
[Patent Document 2] Japanese Laid-open Patent Application No. 2000-219752
[Patent Document 3] Japanese Laid-open Patent Application No. 59-120428
[Patent Document 4] Japanese Laid-open Patent Application No. 60-259430
[Patent Document 5] Japanese Laid-open Patent Application No. 8-267571
[Patent Document 6] Japanese Laid-open Patent Application No. 50-56453
[Patent Document 7] Japanese Examined Patent Application No. 58-188626
[Patent Document 8] Japanese Laid-open Patent Application No. 52-2949
[Patent Document 9] WO2004-067260
[Patent Document 10] WO2004-067266